This was the first day that I saw this in flower and there may be more to come. It does seem to be spread over a small area and I saw 6 to 8 individual plants, so it could have been and escapee that propogated itself. I will go back and add improved photographs showing more of the plant.

I have had a little trouble uploading large photographs, but I wanted to allow as much detail as possible. I did think at first that it might have been 'Sulphur Cinquefoil' as most things fitted except the flower colour.
Over a stretch of about 70 metres on the clifftop and about a two metre width, there were 63 in flower and probable three times that in bud.